A knowledge of the thermal properties of biomaterials has long been considered important to researchers and others interested in increasing man's understanding of the nature of materials and their thermal interactions, as well as to designers of equipment and systems in which the thermal characteristics of the materials used therein or operated thereon are of significance. For example, important information concerning biological materials, such as human and animal tissues, can be obtained from knowledge of the thermal properties thereof.
Thus, it is known that biomaterials are capable of heat transfers by virtue of a temperature gradient, such heat transfer capability being especially important in living biomaterials because the state of life thereof, for example, may depend on the maintenance of a specific temperature level. Heat transfer by conduction is usually most important in determining the heat transfer within the biological medium and such heat transfer is best characterized in the steady-state by the thermal conductivity, K, of the medium and in the non-steady state of its thermal diffusivity, .alpha.. Since there is no presently known method of determining K and .alpha. of a biomaterial from a knowledge of some other fundamental property or properties thereof, it is necessary to devise appropriate processes and apparatus to measure K and .alpha. in some appropriate manner. Accordingly, there has been an increasing utilization, particularly in medical research and clinical laboratories, of processes which require heat transfer through biological materials, such as in cryobiology (e.g., cyrosurgery), in tissue and organ preservation, and in frostbite studies, for example. Other procedures which are heat transfer dependent and, thus, require a knowledge of thermal properties include clinical applications of heated gases or liquids, ultrasonic wave energy, microwave energy and laser beam energy in both diagnostic and therapeutic operating modes - examples being laser surgery and hyperthermia thermal therapy, as an emerging modality for the treatment of cancer.
Such processes require more extensive and more reliable information concerning the thermophysical properties of such materials and, in particular, information concerning the thermal conductivities and thermal diffusivities thereof which permit the determination of temperature distributions, heat transfer rates and, in turn, the flow rates of fluids through the biological medium. Perfusion, the volume flow rate of blood per unit mass or volume of tissue is a primary factor in the local transport of heat, oxygen, drugs and nutrients - thus in the maintenance, assessment and medical intervention of life processes. It is particularly important, for example, to monitor the flow rate of blood through tissue so that flow disturbances can be monitored and corrective action taken in cases where maldistribution of blood flow in a patient would have unfavorable and possible fatal consequences. For example, it would be desirable to provide information during open heart surgery as to blood perfusion rates in the myocardium to assess the success of revascularization procedures and the existence of small vessel disease so that subsequent incisions are not required after primary arterial blood flow correction. Furthermore, with some patients, it is necessary to determine whether further medical assistance to the patient is required. In addition, such information is essential in treating patients afflicted with ulcerated extremities and in the most efficacious application of transcutaneous gas measurements.
Techniques which have been applied to the measurement of thermal properties of biological materials have included both invasive and non-invasive techniques. A general summary of such techniques and the limitations thereof is presented in the text, Annual Review of Biophysics and Bioengineering, "Theory Measurement and Application of Thermal Properties of Biomaterials," H. Frederick Bowman et al, pp. 43-80, Vol. 4, 1975. A review of the most promising thermal techniques (invasive and non-invasive) in the measurement of perfusion is presented in the text, Heat Transfer in Medicine and Biology, Vol. 2, "Estimation of Tissue Blood Flow," Chapter 9, H. Frederick Bowman, Plenum Publishing Corporation, 1984.
Knowledge of perfusion is generally known to be of importance in the clinical evaluation of patients, in selecting therapeutic interventions and in general patient management. Altough measurement of perfusion in deep tissue, that located greater than about 0.5 cm from the tissue surface, generally requires invasive procedures and devices (Bowman Patent No. 4,059,982; Bowman, ibid, and general literature reviews), several important clinical and research problems require the accurate measurement in the outer layer of tissue surface. Examplary problems are the measurement of: epicardial perfusion following open heart surgery, rate of revascularization of burn patients, requirement of and/or location of where to amputate ulcerated extremities. Sitll another important problem is the measurement of skin perfusion during transcutaneous blood gas monitoring, wherein it would be desirable to determine non-invasively both perfusion and the temperature of the blood in the capillaries which are located within about 100 to 300 microns of the skin surface, without which the transcutaneous measurements are inaccurate, such that present devices and methodology are generally not capable of providing transcutaneous measurements on adults. In general, therefore, it is important to have non-invasive means for accurate measurement of tissue perfusion in the layer of tissue located within the first several millimeters of the tissue surface.
While prior non-invasive techniques can provide some information about surface region perfusion, they have been limited in that the information obtained is generally qualitative and inaccurate, since the thermal coupling between the measuring device and the system being measured is not accounted for and since means for thermally guarding or actively shielding the surface sensor are not provided, with the consequence thereof being that the quantitative methodology exemplified in the use of the thermal diffusion probe (Bowman, U.S. Pat. No. 4,059,982; and reviews entitled "Theory, Measurement and Application of Thermal Properties of Biomaterials" and "Estimation of Tissue Blood Flow," as aforementioned), cannot be used. Thus, generally speaking, accurate measurement of perfusion has been limited to invasive means, during which the sensor(s) are surrounded completely by and fully exposed to the influence of the very medium, the tissue or biomaterial, for which the perfusion measurement is sought. Such invasive techniques involve the implantation within the specimen material of heat sources (or sinks) which may also serve as temperature sensors. Probes which have been utilized for this purpose include the thermal comparator, the heated thermocouple and the heated thermistor. However, invasive probes are undersirable since they cause discomfort, trauma to the patient and danger of infection.
Accordingly, it would be highly desirable to provide a non-invasive means for measuring blood perfusion in order to reduce patient trauma and risk of infection while providing an accurate means of blood perfusion through characterizing the influence of the measurement device, while avoiding the deleterious influences of the ambient thermal environment, in order to reduce patient trauma and discomfort while providing an accurate means for measuring blood perfusion in the surface layer of tissue.